The magnetic field of the NGC 2024 molecular cloud

We have carried out Very Large Array (VLA)(1) Zeeman observations of absorption lines of H I and OH toward the molecular cloud associated with the NGC 2024 (Orion B) H II region. The synthesized beam diameters are 68 " x 52 ", p.a. = 38 degrees, and 81 " x 65 ", p.a. = -6 degrees, for OH and H I, respectively. The absorption lines could be mapped over the NGC 2024 continuum source, which has an extent (at the 1 Jy beam(-1) level) of Delta alpha approximate to 7' by Delta delta x 5'. The maps of the magnetic field, together with comparisons with additional data from the published literature, lead to the following conclusions: (1) The magnetic field comes from a line subcomponent at upsilon(LSR) approximate to 10.2 km s(-1), which corresponds in velocity and in spatial morphology with the northern dense molecular ridge in NGC 2024. (2) B-los varies from 0 to the northeast of the northern molecular ridge to almost 100 mu G to the southwest. The variation in B-los may be due to the field being mainly in the plane of the sky to the northeast but having a significant line-of-sight component to the southwest. (3) Velocities in the cloud are supersonic but approximately equal to the Alfven velocity, which is consistent with motions being dominated by magnetohydrodynamical waves rather than thermal motion or hydrodynamical turbulence. (4) The mass-to-magnetic flux ratio is supercritical, which suggests that the static magnetic field does not support the cloud against collapse. Simple virial estimates of the relative importance of gravitational, kinetic, and magnetic energies show that the ratio of kinetic/gravitational energy is about 0.5, while the magnetic/gravitational energy ratio is less than 0.1. At face value, these results imply that the cloud is supported mainly by nonthermal motions rather than by the static magnetic field. However, since we only measure directly the line-of-sight component of B, this result is not conclusive.